The data to be written to the stream. Although the buffers object
may be copied as necessary, ownership of the underlying buffers is
retained by the caller, which must guarantee that they remain valid
until the handler is called.

handler

The handler to be called when the write operation completes. Copies
will be made of the handler as required. The equivalent function
signature of the handler must be:

voidhandler(constboost::system::error_code&error,// Result of operation.std::size_tbytes_transferred// Number of bytes written.);

The async_write_some operation may not transmit all of the data to the
peer. Consider using the async_write function if you need
to ensure that all data is written before the blocking operation completes.